1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens. The present invention is suitable, for example, for digital cameras, video cameras, and film cameras.
2. Description of the Related Art
Recently, in image pickup elements used in image pickup apparatuses, such as digital cameras, the number of pixels has been increasing. Accordingly, photographing lenses (image pickup optical systems) for use in image pickup apparatuses including image pickup elements with large number of pixels are required to have a high resolution.
In high-resolution photographing lenses, various aberrations, such as a spherical aberration and a coma aberration, that affect monochrome image quality must be accurately corrected.
In addition, chromatic aberrations must also be accurately corrected so that color bleeding does not occur in an image obtained when white light is used as illuminating light.
Also, to obtain a large imaging area, zoom lenses with a high zoom ratio are desirable for photographing lenses. In general, as the focal length at the telephoto end is increased in order to increase the zoom ratio, chromatic aberration of magnification is increased at the wide-angle zoom position, and both chromatic aberration of magnification and axial chromatic aberration are increased at the telephoto zoom position.
Therefore, in order to achieve a high image quality, it is important to perform accurate correction not only for a first-order spectrum but also for a secondary spectrum.
Generally, various structures for correcting the secondary spectrum of chromatic aberrations at the telephoto end are known. In such a structure, among lens units included in a zoom lens, a lens unit near an object side includes a lens formed of anomalous partial dispersion glass.
For example, U.S. Pat. No. 7,088,521 discusses a structure in which chromatic aberrations are corrected by placing a replica layer (refractive optical element) having anomalous partial dispersion characteristics in a zoom lens.
Zoom lenses that correct chromatic aberrations using a diffractive optical part (diffractive optical surface) instead of using an anomalous partial dispersion material are also known.
For example, zoom lenses are known which include a plurality of diffractive optical parts arranged in front of and behind an aperture of the optical system (Japanese Patent Laid-Open No. 2004-117826, U.S. Pat. Nos. 6,275,342, and 6,081,389).
Also, zoom lenses having a four-lens-unit structure including, in order from the object side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power are known.
U.S. Pat. Nos. 6,404,561 and 6,594,087 discuses zoom lenses that have the above-described four-lens-unit structure and that correct chromatic aberrations using an anomalous dispersion glass.
Zoom lenses having a five-lens-unit structure including, in order from the object side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power are also known (U.S. Pat. No. 6,025,962).
In general, when a zoom ratio of a zoom lens is increased, variation and magnitude of chromatic aberrations that occur during zooming are increased. In particular, the secondary spectrum of axial chromatic aberration and chromatic aberration of magnification is increased at the telephoto side and it becomes difficult to accurately correct the secondary spectrum.
The chromatic aberrations at the telephoto side can be reduced by an anomalous dispersion effect obtained by placing a diffractive optical part in a lens unit near the object side.
However, when the chromatic aberrations at the telephoto side are corrected by placing a diffractive optical part in a lens unit near the object side, the chromatic aberrations at the wide-angle end cannot be effectively corrected by the diffractive optical part. On the contrary, if the refractive power of the diffractive optical part is increased to greatly reduce the chromatic aberrations at the telephoto side, the chromatic aberration of magnification at the wide-angle end is increased.
In comparison, the chromatic aberrations can be easily corrected at both the wide-angle end and the telephoto end by placing a plurality of diffractive optical parts in the optical system or by placing an optical component having high anomalous dispersion characteristics near the image plane side.
In general, diffraction efficiency of a diffraction grating varies in accordance with the incidence angle of light rays. Therefore, in a zoom lens with light-ray incidence conditions that vary during zooming, there is a risk that the diffraction efficiency will be reduced during zooming unless the position at which the diffraction grating is disposed is adequately set.
In addition, if the chromatic aberrations of the optical system are to be corrected only by the diffractive optical part, it is difficult to increase the image quality by correcting the chromatic aberrations over the entire zoom area while increasing the zoom ratio.
To obtain high optical performance by accurately correcting the chromatic aberrations over the entire zoom area while increasing the zoom ratio, it is important to adequately set the positions of the diffractive optical part and the lens made of anomalous partial dispersion material, optical characteristics thereof, etc.